1. Field of the Invention
The inscription and marking of materials by means of laser is widespread; it involves the removal of material. In the case of uniform material it constitutes etching; if, however, a thin top layer of different colour is removed, then high-contrast inscriptions can be obtained, as is the case in the production of frontplates in day-and-night design for car radios. Similar techniques are also known for two-dimensional structures such as signs and labels. Two-dimensional laser-inscribable materials of this kind are distinguished by a two-layer or multilayer construction comprising thin top layers and thick base layers, with the individual colour layers being intended to exhibit maximum contrasts.
2. Description of the Prior Art
In accordance with DE U 81 30 861, the coating layers consist of solvent-free, electron beam-cured coating materials which are applied in succession. The resulting film material is provided with an additional layer of adhesive. The high-quality two-layer or multi-layer film material is distinguished by high temperature resistance, weathering resistance and chemical resistance. By means of the laser beam it is possible to remove the top layer selectively and, owing to the colour contrast of the base layer, a readily visible inscription (letters, numbers, symbols, logos, etc.) is produced.
DE 41 34 271 describes the production of a composite material comprising two coloured coating layers of different colours, with at least the top layer which is to be subjected to laser being applied by the transfer method. An advantage stated here is the high uniformity which can be achieved in layer thickness.
In DE 39 25 563, a composite material comprising a glass fibre mat and a black PTFE coating is used as base material for flexible, temperature- and chemical-resistant product labels which can be written on by means of laser.
All of the above multilayer systems are characterized by a complex method of production--in addition to the production of the base support, which is usually a film, it is necessary in a second operation to apply an appropriate top layer which is different in colour. For reproducible, well-defined inscriptions, stringent requirements must be placed on the layer thickness tolerances which requirements, especially in the case of coating techniques such as for DE U 81 30 861, imply apparatus of considerable complexity and restrict productivity. In many cases, interlaminar adhesion between the successively applied layers constitutes a weak point--only by using special production techniques and/or additives in the formulations is it possible to improve the bond strength to such an extent that the material can be employed even for high-performance applications.
In the two-layer systems mentioned above, an inscription is obtained by removing the upper layer, termed the top layer. Given an appropriate choice of the materials for the polymer structure and of the colours/pigmenting of the top and base layers, it is possible to obtain high contrast and high definitions of inscription. The supply of highly concentrated electromagnetic radiation, mostly in the infrared region, leads to a discharge of material at the point where the laser beam impinges, in the form of aerosols. For reasons of workplace safety the area where laser inscription is carried out has to be exhausted and even, where possible, isolated; the exhaust air is generally cleaned by way of particle filters and absorption filters in order to prevent pollution of the environment with in some cases toxic splitting and elimination products.
The object of the invention was to remedy this state of affairs, and in particular to provide a laser label of simple construction from which the disadvantages of the prior art are absent or in which they are at least reduced, but which nevertheless possesses advantageous properties in use.